A very important property of lasers is that they typically have a relatively long coherence length (or, equivalently, coherence time), as compared to standard incoherent sources. This is a very important property when a given wavefront which is produced by the same source at a later time, such as in constructing an interferogram with an unequal path length interferometer or in many applications of holography. Laser sources having coherence lengths of from meters to kilometers are well known. These devices are generally of low power (milliwatts to watts), utilized for photographic exposure techniques applicable to holography or interferometry.
Coherence is also an important factor for certain high energy lasers. This can be a result of using large optical elements required to focus the high energy laser (HEL) beam over hundreds to thousands of kilometers down to a reasonably small diameter (tens of centimeters) on the target of interest. The diameter, d, of a circular mirror which focuses down to a spot size of X at a distance Z, for a wavelength .lambda. (all expressed in meters) is given by the expression d=2.44 (.lambda.Z)/X. As an example, for a wavelength of 1 micrometer, a focal distance of 1,000 kilometers, and a focal spot of 30 cm (0.3 meters), the mirror diameter, d, is 8.13 meters. Mirrors of this size are currently conceived of as being segmented, i.e., composed of several separate segments, each of which is flat or nearly flat. Each segment is independently directable to a given target and each segment must be phase matched to every other segment for optical focusing on a target.
Phase matching of segments can occur even with a beam which is incoherent provided (1) each segment is illuminated with a portion of the beam from the same single source, and (2) the path length from the source to each mirror segment is the same. For some applications one wished to relax requirement (1); this may lead to requirements that each output source be individually coherent (have a relatively long coherence length) and that mutual coherence of the various sources be achieved by some special means of phase locking. Possible methods of phase locking of physically separate radiation sources have been considered by various workers, but are not the subject of this invention.
Accordingly, there is a need for an invention which is concerned with situations in which condition (2) named above is violated, i.e., situations in which the path length from the source to various output mirror segments is not necessarily the same. In order that the beams from various segments combine coherently and be focusable to a high degree, it is then essential that the beams falling on each segment shall individually have relatively long coherence lengths (at least as long as the differences in path lengths to various mirror segments). In the situations encountered prior to the invention of RF Linac Free Electron Lasers, this requirement amounted to requiring a laser type source or sources in order to produce a coherent beam, and in the case of multiple lasers required means of phase locking the various lasers together. A different problem arises from RF Linac FELs. For brevity, these will sometimes be referred to as RFFELs.
An inherent property of RFFELs is that high energy electrons which pass through the wiggler magnets producing laser gain, and also the photons involved in the lasing action, consist of extremely short micropulses (typically 30 picoseconds) which are spaced apart in time by a somewhat larger interval than the micropulse width, but one which is nevertheless rather short. A typical time between pulses is 10 nanoseconds. Considering that the photons travel essentially at the speed of light in vacuum and that the electron bunches travel at a speed only very slightly less, it follows that there are micropulses of photon of length of typically 0.9 centimeters, separated by intervals (micropulse separation distance) of typically 300 centimeters. We primarily consider here operation as a ring resonator, though the general situation would also apply in a standing wave resonator. The overall design must be such that the round trip path length (of photons) in the resonator is an integer multiple of the micropulse separation distance, in order that the relatively short micropulses of gain in the wiggler be present at just those intervals of time when a micropulse of photons passes through. (For brevity we designate this integer by N). This is well recognized and incorporated in designs. If the integer multiple, N, were simply unity, the problem addressed in this invention would not arise.
A particular problem arises for RFFELs because the round trip path length in the resonator is, for practical reasons, somewhat longer than the micropulse separation time; hence the integer N is larger than unity (perhaps falling between 10 and 100). It is inherent in the operation of a resonator that there will be mutual coherence of any set of micropulses which are separated by a multiple of N. Hence there are sets of micropulses such that each set is coherent within itself. The micropulses thus become naturally (logically) separated into sets of micropulses, such that each set is coherent, but there does not exist mutual coherence between members of any two distinct sets of micropulses. As a simple example the set of micropulses numbered (1, N+1, 2N+1. . .) is not coherent with the set of micropulses numbered (2, N+2, 2N+2, . . .). The purpose of this invention is to arrange that various such sets of micropulses are mutually coherent. The practical importance is that output mirror segments can be fed by beams from the RFFEL which differ in path length by the micropulse separation interval (typically some 3 meters), or any integer multiple thereof. Without use of this invention the path length difference between beams to separate mirror segments would have to be an integer multiple of the resonator round trip length, which is typically much larger (perhaps many tens of meters).